Hi Diodes rarely fail half way. Either a bond wire goes out or they melt down. If the varicap looks like a diode at DC it should be ok at RF. I would look elsewhere for the problem.
Bob On Jan 15, 2010, at 11:07 PM, Neville Michie wrote: > Hi, thanks for the constructive comments. > > I took both units out of their cases and gave them a visual inspection. > I tested the varicap diode as a diode, and checked the values of surrounding > resistors. > When I re-assembled them one of them now works OK. It will take some time to > see if it is stable. > I have an instrument that I can run them in which has an EFC lead that I can > easily lift, > so I am going to apply about 20V through 1megohm to the EFC terminal and see > how high the voltage measures, > that should give me the 6.8 volts regulated plus a diode drop. The diode and > the capacitor should stand 20V. > I dont like trying to run it while dissasembled, that can lead to disasters, > but this test should show if > the 6.8 volts is a problem. Failing that I will unsolder the varicap > and measure its capacitance on a bridge with changing bias voltage. > Thanks again for the suggestions, > > Neville Michie > > > On 16/01/2010, at 11:54 AM, Rick Karlquist wrote: > >> No one has mentioned the fact that the cathode of the varactor >> is connected to an internal 6.8V Zener diode voltage reference. >> This is another source of trouble. >> >> Rick Karlquist >> N6RK >> >> >> Bob Camp wrote: >>> Hi >>> >>> I sure would do some quick continuity checks on the EFC line before I >>> started swapping varicaps out. A bum solder joint opening up is a lot more >>> likely than the varicap going nuts in a fashion that still lets the >>> oscillator run. >>> >>> Bob >>> >>> >>> On Jan 15, 2010, at 7:08 PM, Neville Michie wrote: >>> >>>> Hi, >>>> >>>> I have two HP10811 oscillators that do not respond to EFC voltage. >>>> The latest to fail had been quite accurately set against a TBOLT, >>>> one morning (after being on standby in a HP5328A) it was off frequency >>>> and would not shift with EFC but would adjust with the coarse adjust. >>>> I have bought in two Phillips varicap diodes, BBY40 and BB149A, >>>> but before I did some damage fault finding in the HP10811s I thought >>>> it wise to ask for any advice that was available. >>>> While I am playing with them I may try to readjust the temperature >>>> setting >>>> of the oven to get on top of the turnover. Can anyone point me to that >>>> article? >>>> >>>> cheers, Neville Michie >>>> >>>> _______________________________________________ >>>> time-nuts mailing list -- [email protected] >>>> To unsubscribe, go to >>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>> and follow the instructions there. >>>> >>> >>> >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >>> >>> >> >> >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
